The present invention relates to a Fresnel lens and to an illuminating device comprising the Fresnel lens and an LED as a light source for using in a portable telephone, electrical appliances and others.
The illuminating device for illuminating displays of portable telephones and small electrical appliances comprises a light source such as an LED or lamp and a semispherical lens or Fresnel lens. Since the Fresnel lens has a thin thickness, the illuminating device can be made into a small size.
There is known two types of Fresnel lenses. One of the types is the refraction type, the other is the reflection type.
FIG. 8 is a plan view showing a refraction type Fresnel lens, FIG. 9 is a sectional side view of an illuminating device using the Fresnel lens, FIG. 10 is an enlarged view of an A-portion of FIG. 9, and FIG. 11 is an enlarged view of a B-portion of FIG. 9.
The Fresnel lens 1 comprises a central convex lens 2 formed at a central portion of an upper surface of a lens plate member 1a, and a plurality of annular circumferential lenses 3 coaxially disposed with the convex lens 2. Each of the circumferential lenses 3 has a sectional shape of a prism comprising an outside semispherical refraction face 3a and an inside cylindrical face 3b as shown in FIG. 10.
The sectional area of the prism reduces toward the periphery. Heights of the convex lens 2 and circumferential lenses 3 are substantially equal with each other and inclination angle of the refraction face 3a increases toward the periphery so that the distance between adjacent prisms reduces toward the periphery.
An illuminating device 4 shown in FIG. 9 comprises a box 5 supporting the Fresnel lens 1 and an LED 6 mounted on a bottom 7 of the box 5.
Light rays 8 emitted from the LED 6 are slightly refracted when entering the lens plate member 1a and further refracted when exiting from the lenses. Angles of refraction faces 3a are set so that the light rays 8 exit the Fresnel lens in parallel as shown in FIG. 9.
The reflection type Fresnel lens is disclosed in Japanese Patent Application Laid Open 2001-337206.
FIG. 12 is a plan view showing a reflection type Fresnel lens, FIG. 13 is a sectional side view of an illuminating device using the Fresnel lens, FIG. 14 is an enlarged view of a C-portion of FIG. 13, and FIG. 15 is an enlarged view of a D-portion of FIG. 13.
The Fresnel lens 10 comprises a plurality of annular lenses 13 coaxially formed on an underside of a lens plate member 11. Each of the annular lenses 13 has a sectional shape of a prism comprising an outside cylindrical incident face 13a and an inside inclined reflection face 13b as shown in FIG. 15.
The sectional area of the prism increases toward the periphery. Inclination angle of the reflection face 13b reduces toward the periphery so that the distance between adjacent prisms increases toward the periphery.
An illuminating device 14 shown in FIG. 13 comprises a box 15 supporting the Fresnel lens 10 and an LED 16 mounted on a bottom 17 of the box 15.
Light rays 18 emitted from the LED 16 are refracted when entering the cylindrical incident face 13a and upwardly reflected on the reflection face 13b. Angles of reflection faces 13b are set so that the light rays 18 exit the Fresnel lens in parallel as shown in FIG. 13.
Although the Fresnel lens can be made into a small size as described above, there are problems that the process for manufacturing Fresnel lens of small size is difficult. The inclination angle of the prism becomes large in the peripheral area in the refraction type lens as shown in FIG. 10, and in the central area in the reflection type lens as shown in FIG. 14, thereby increasing density of the prisms in both areas. The Fresnel lens is manufactured by molding a resin in a mold. The mold is formed by a cutting tool. In order to cut a metal to form a prism having such a large inclination angle, the vertex of the cutting tool is very small, and hence the width of the cutting tool is very small. The tip of such a cutting tool is liable to be broken. Consequently, it is difficult to manufacture the mold for Fresnel lens. Namely, there is a manufacturing limitation in vertex of the prism.